The present invention relates to cartridge-operated fastening tools and, more specifically, to the type of such tools in which a fastener is driven into hard materials by a captive piston adapted to be propelled by the combustion gases of an explosive load, for instance a blank cartridge.
Such tools are known in the art. They have been described, for example, in British Pat. Nos. 734,344, 834,293, 901,023 and German Pat. No. 1,224,218. In the above patents, the fasteners are guided in differring ways. In Pat. No. 834,293, a stud is assembled in a two-piece piston. This is a lengthy operation. Further, it is necessary, to separate the two pieces, of the piston, after the tool has been fired. No realistic guidance means are proposed for nails. At best, it is necessary to use a different piston when the stud diameter is different. The disadvantages are such that this type of tool has been abandoned.
In German Pat. No. 1,224,218, the centering means comprise a recess in the piston for the fastener head and a plastic washer, flexible enough to slide in two different diameter bores. A movable steel washer may be carried on the fastener shank, proximate the nose. This method does not yield a quality fixing. Further, a special ramrod is necessary to load a new fastener in the tool and return the piston to the firing position. Therefore, although these means lead to light pistons and short tools, this type of tool has not been successful.
In British Pat. Nos. 734,344 and 901,023, the fasteners have two centering means. In both cases, a muzzle bushing is provided, for co-operation with a steel or plastic washer assembled on the fastener shank. In the British Pat. No. 734,344, the second means comprises the fastener head, or a cap on it, co-operating with the same muzzle bushing. In Pat. No. 901,023, the second centering means comprises a recess in the front part of the piston, in which the fastener head or a rear extension is inserted.
In both patents, the front part of the piston is centered in the muzzle bushing prior to any displacement and, consequently, the length of the portion of the piston shank which is slidable in the muzzle bushing must necessarily be somewhat greater than the length of the longest fastener usable in the tool. This results in serious limitations on usable fastener lengths, as long fasteners, for instance 4 or 5 inches long, would require an excessively long tool and heavier pistons than desired.